1. Field of the Invention
This invention relates to heat exchanger apparatus for use with a boiling liquid. More particularly this invention relates to a heat exchanger tube having a fluid to be cooled passing therethrough and a boiling refrigerant in contact with the external surface of the tube. The invention also relates to the method of manufacturing a tube of this particular configuration.
2. Prior Art
In certain refrigeration applications such as a chiller or an evaporator liquid to be cooled is passed through a tube while liquid refrigerant is in contact with the outside of the tube. Usually the tube is either immersed in refrigerant or wetted with a refrigerant spray. The refrigerant changes state from a liquid to a vapor absorbing heat from the fluid to be cooled within the tube. The selection of the external configuration of the tube is extremely influential in determining the boiling characteristics and overall heat transfer rate of the tube.
It has been found that the transfer of heat to a boiling liquid is enhanced by the creation of nucleate boiling sites. It has been theorized that the provision of vapor entrapment cavities in the heat exchanger surface creates sites for nucleate boiling.
In nucleate boiling the trapped vapor is superheated by the heat exchanger surface and consequently grows in size until surface tension is overcome and the vapor bubble breaks free from the surface. As the bubble leaves the surface, liquid wets the now vacated area and the remaining vapor has a source of additional liquid for creating vapor to form the next bubble. The continual wetting and release together with the convection effect of the supeheated bubble traveling through and mixing the liquid result in an improved heat transfer rate for the heat exchanger surface.
It is known that the surface heat transfer rate is high in the area where the vapor bubble is formed. Consequently, the overall heat transfer rate tends to increase with the density of vapor entrapment sites per unit area of heat exchanger surface. See for example, U.S. Pat. No. 3,696,861 issued to Webb and entitled "Heat Transfer Surface Having A High Boiling Heat Transfer Coefficient," or Heat Transfer by M. Jakob, vol. 1, published by John Wiley and Sons.
There are numerous heat transfer surfaces which utilize nucleate sites to enhance overall heat transfer rates. In U.S. Pat. No. 3,454,081 granted to Kun and Czikk entitled "Surface For Boiling Liquids," a cross-grooved boiling surface layer is created having sub-surface cavities with restricted openings to the outer surface of monoscopic density. In U.S. Pat. No. 3,326,283 issued to Ware and entitled "Heat Transfer Surface," fins on tube are deformed to form indentations for the promotion of nucleate boiling.
There are also many methods of creating nucleate boiling surfaces. In U.S. Pat. No. 3,487,670 entitled, "Method of Forming Indentations In Fins Extending From A Heat Transfer Surface," a method is disclosed of forming the heat transfer surface in the Ware patent above. The fins are rolled with an indenting tool which flares the fin material beyond each side wall of the fin to form the vapor entrapment cavity. In U.S. Pat. No. 3,496,752 granted to Kun the method includes scoring the heat transfer surface to form grooves of microscopic density and then forming cavities by deforming the material between the grooves into the grooves. In Webb, U.S. Pat. No. 3,696,861, fins on a heat exchange tube are unidirectionally rolled over toward the adjacent fin to form vapor entrapment sites therebetween.
The creation of a cost effective high performance (nucleate boiling) heat exchanger tube that can be manufactured from a commercial tube blank in a single pass on a conventional tube finning machine is the problem resolved herein. In order for the tube to be cost effective, the additional expense in manufacturing the high performance tube must be recovered either in the decreased expense of construction utilizing the higher performance tube or in increased overall capacity of the heat exchanger.